use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestInterface;

class CustomPrice implements ObserverInterface
{
    public function execute(MagentoFrameworkEventObserver $observer) {
        $item = $observer->getEvent()->getData('quote_item');

        $item = ( $item->getParentItem() ? $item->getParentItem() : $item );
        $price = 100;
        $item->setCustomPrice($price);
        $item->setOriginalCustomPrice($price);
        $item->getProduct()->setIsSuperMode(true);
    }

}